1. Field of the Invention
The present invention relates to a multilayer ceramic substrate having a cavity, and more particularly, to an improved multilayer ceramic substrate with a cavity having a reduced thickness.
2. Description of the Related Art
FIG. 8 is a cross-sectional view showing a multilayer ceramic substrate 1 with a cavity that is manufactured by conventional techniques.
The multilayer ceramic substrate 1 with the cavity is designed such that it may be mounted on a mounting mother board 2 represented by the hatched line. The multilayer ceramic substrate 1 with the cavity includes a multilayer composite member 4 having a plurality of ceramic layers 3 disposed one on another. A cavity 7 is formed in the multilayer composite member 4 such that the cavity 7 has an opening 6 located in at least one principal surface of the multilayer composite member 4 (for example, in a principal surface 5 facing the mounting mother board 2). An electronic component 8 in the form of a chip, such as an integrated circuit or a surface acoustic wave filter, is disposed in the cavity 7.
Although not clearly shown in FIG. 8, a plurality of inner conductive films and a plurality of via-holes are formed in the multilayer composite member 4 so as to provide interconnection lines needed in the multilayer ceramic substrate 1. As required, some of those inner conductive films and/or via-hole conductors are arranged so as to provide a capacitor, an inductor, a delay line, and/or a filter. In some cases, in addition to the inner conductive films described above, a resistive film is disposed in the multilayer composite member 4 so as to provide a resistor.
In the multilayer composite member 4 shown in FIG. 8, a pair of capacitor conductive films 9 and 10 is arranged such that they face each other through one of the ceramic layer 3, thereby defining a capacitor.
In some cases, although not shown in FIG. 8, external conductive films are formed on the other principal surface 11 of the multilayer composite member 4 opposite to the principal surface 5 in which the cavity 7 is formed, and another electronic component such as a capacitor, an inductor, a resistor, a diode, an integrated circuit, or a memory is mounted using the external conductive films.
In many cases, a bottom-surface conductive film 13 is formed on the bottom surface 12 of the cavity 7. The chief ingredient of the bottom-surface conductive film 13 is a metal. This allows the bottom-surface conductive film 13 to have a color different from that of the ceramic layers 3 or the electronic component 8. The difference in color makes it easy to optically detect the location of the electronic component 8 when the electronic component 8 is put into the cavity 7. That is, one of functions of the bottom-surface conductive film 13 is to make it easy to detect the location of the electronic component 8.
In general, a ground potential is applied to the bottom-surface conductive film 13. In this case, when an electronic component 8 is needed to be grounded, the bottom-surface conductive film 13 is electrically connected to the electronic component 8 to supply the ground potential to the electronic component 8.
Regardless of whether the ground potential is applied to the bottom-surface conductive film 13, if the electronic component 8 is not needed to be electrically connected to the bottom-surface conductive film 13, the electronic component 8 may be connected to the bottom-surface conductive film 13 via a nonconductive adhesive.
There has been a desire to further reduce the size of electronic devices. In order to satisfy the desire, it is required to reduce the size of various electronic components used in electronic devices. Thus, it is also desirable to reduce the size of the multilayer ceramic substrate 1 with the cavity. In particular, a reduction in the total thickness of the multilayer ceramic substrate 1 with the cavity is very desirable.
The reduction in the total thickness of the multilayer ceramic substrate with the cavity 1 shown in FIG. 8 can be achieved by applying preferred embodiments of the present invention.